Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwinstonwood on April 29, 2020, 06:54:57 pm
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Hello all,
I'm new here, this is my first post, I'm David.
I'm interested in building my second tube amp. I just finished building an EL34 SET stereo hifi amp - my first ever - that works and sounds great. I had a lot of help on another forum.
I really enjoyed that build, and I'd like to start planning my next project. I really don't want to build a 5F1, or 5E3, etc. I'd like to do something a bit different.
What I'm looking at is the Vox AC4 (V-1-1). However, I'm wondering if it's possible to build it without the tremolo part of the circuit. I've attached the schematic below. I don't know enough about amp circuits to know how much the tremolo circuit interacts with the rest of the amp. I can see that it is connected to the V2 cathode bias for one thing, among others. If I was to build the amp without the tremolo where would I cut out the circuit, and would I need to alter the values of any other components?
Thanks in advance! I really think this would make a cool little 3-tube EL84 amp that's different enough from Champs to have its own sound.
David
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dave also :icon_biggrin:
think this works, but wait for confirmation
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Thank you Dave!
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Shooter got it right but he forgot to cut out the "witch". :l2:
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Thanks sluckey!
I'm going to give this project a shot and start making a bom.
David
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After studying this circuit, I've realized that it's not the best project for a beginner. The rectifier is expensive and uncommon, and I have no idea how to select the best OPT, as well as some of the capacitors.
I'm teaching myself LTSpice (I've been using it less than a week) by copying existing schematics. This schematic has been very frustrating.
So, I'm switching my focus onto the Schedule 40 GA-5. There is a lot more available info on this project, and it still uses an EL84 like the AC4.
First, I'm going to draw the circuit in LTSpice. After hours of trying to understand how to link .SUBCKT text files to an existing component such as the pentode in the misc folder, I discovered that I can open the text file in LTSpice, right-click on the Name line and let the program create the component for me!
I will also continue reading everything on this forum about the Schedule 40, and learn all I can on my own before posting a bunch of questions. :icon_biggrin:
Thanks,
David
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I have no idea how to select the best OPT
the tube datasheet holds the key, in this example ~ 5k
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> The rectifier is expensive and uncommon, and I have no idea how to select the best OPT
Yeah. If you get it wrong the Earth will turn backward and we will all fly off into space.
BAH! Rectifiers are generic and not sound-shapers. As said there's DATA in datasheets. And this amp was NOT designed in simulation. It was a guy with various parts and a hot soldering iron. (It is really a cross between a good radio and a small PA amp.)
And it is really a Fender Champ with different tubes. Champ transformers WILL work. That US rectifier we like on Champs is all the same to the tone. The EL84 needs a different socket and a *different* cathode bias resistor, just get the right parts. That pentode preamp echos the early Champs but you may as well use the VOX tube and values so "it isn't a Champ".
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...think this works, ...
C5 should stay with the "260V" point (https://el34world.com/Forum/index.php?action=dlattach;topic=25921.0;attach=83322;image). It may work without it, but it is Good Practice.
By logic, R7 should be 1.5x-2.0x larger without that 100k dumping into it. I bet it works without this adjustment. The noted DC voltages will be different. But pentode voltages can vary a lot and still work fine.
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Thanks PRR. I'll amend my schematic.
I'm sensing encouragement to stay with the AC4. So, can I just use a "Champesque" 5Y3GT?
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So, can I
Yep, can also use SS bridge, diodes.....
you just "match" the transformer to the load's requirements, walla DC power :icon_biggrin:
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The rectifier is expensive and uncommon
I would use an EZ81. Same pin out, higher current capacity, much cheaper, and it can work on the 6.3vac filament string just like it's little brother EZ80.
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Thanks sluckey.
I will look at the data sheets for the three tubes: EZ81, EF86, and EL84, and see if I can figure out how many amps a PT's 6.3v windings will need to supply power to all three tube's heaters. I think I saw somewhere that the EZ81 should have 1A. For all three tubes, I'm guessing I'll want to choose a PT that's rated at 6.3V @ 3A minimum, maybe 4A?
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Thanks sluckey.
I will look at the data sheets for the three tubes: EZ81, EF86, and EL84, and see if I can figure out how many amps a PT's 6.3v windings will need to supply power to all three tube's heaters. I think I saw somewhere that the EZ81 should have 1A. For all three tubes, I'm guessing I'll want to choose a PT that's rated at 6.3V @ 3A minimum, maybe 4A?
The EZ81 needs its own separate 6.3VAC winding (i.e. that's two separate 6.3v windings altogether). The EZ81 6V winding wants to be riding on top of the B+ voltage (in order to minimise h-k voltage) - whereas the other 6.3V winding wants to be referenced closer to ground potential (for the same reason).
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Thanks tubeswell. I'll need to read up on that... the part about riding on B+ and heater/cathode voltages went over my head.
Maybe the 5Y3GT on its own separate 5V secondary will be easier for me to comprehend.
I've reached a point with my LTSpice schematic where I want to post what I've done and find out where all of the mistakes are. I could not find a model online for the EF86 that had a fifth pin (for the suppressor grid I guess?) so that component is not going to work in a simulation unless I can figure out how to add it to the .inc file myself.
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> EZ81 needs its own separate 6.3VAC winding .... (in order to minimise h-k voltage)
It's rated 500V H-K. It is clearly intended to be used with heater nearer B-, shared with the other tubes.
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> EZ81 needs its own separate 6.3VAC winding .... (in order to minimise h-k voltage)
It's rated 500V H-K. It is clearly intended to be used with heater nearer B-, shared with the other tubes.
Noted thanks. That aside, would it not be less risky for h-k insulation if it has its own separate filament winding?
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So, this schematic actually runs without error messages in LTSpice.
I have no idea what that means... apparently the app doesn't provide smoke and flame animations.
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Thanks tubeswell. I'll need to read up on that... the part about riding on B+ and heater/cathode voltages went over my head.
The rectifier tube cathode is ‘sitting’ at B+ voltage because it is connected directly to the reservoir filter cap. In an indirectly heated rectifier (like an EZ81), the rectifier’s 6.3vac filament is separate from the cathode, but is squeezed right up against the cathode. So an insulation coating compound is applied to the filament that prevents arcing over between the cathode and the filament. This is called the heater-cathode insulation (‘h-k insulation’). The h-k insulation has a maximum rating which if exceeded, will be likely to cause the insulation to fail. (This risk is not a ‘yes/no’ risk, but more of a higher/lower risk. The higher the voltage differential between the filament and the cathode, the greater the risk of h-k insulation breakdown.) h-k failure here will produce a short between the B+ and the filament winding. You can place your trust in the manufactures 500V max h-k insulation rating, and run the EZ81 of the same filament as the other tube heaters, or you can use a separate 6.3VAC source. Up to you.
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If you want to go traditional, use a vacuum rectifier, but remember that in SE amp you can use Solid State rectifiers without any kind of loss
That is because SE amps idle current Is very close to max power current and B+ did't drop so much like it happens in PPAmps (near NO SAG on SE amps)
Franco
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+1
and it's easy on spice, and it's easy to wire and its easy to find a PT.... :icon_biggrin:
an SE amp has no "Built in" hum suppression like PP, so the supply should be tight, as little AC riding on the B+ as possible. A SS FWB with big caps gets you as close as you can get for the least effort.
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I've seen many indirect-heat rectifiers serve long life with hundreds of volts of stress.
While there is an internet "fact" that these rectifiers are fragile, I never knew that when working with them. I don't doubt that a few have failed, but how many direct-heat rects fail? (Lots!) Large grain of salt.
The insulation is not crap. It should be good for way more than the usual 100V rating. And we know 12AX7 serves in McIntoshes exposed to 400V swings. I think that 99% of non-rect duties do not need more than a few volts, so they do not promise it, and do not obsess about it in production. But knowing that a specific cathode will be sold with a 500V promise, they may do a full or extra spray/dip, and they may do more arc-testing.
I think there are many other things more worrisome.
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and it's easy on spice
Well, I had to do a lot of Googling, and watch a few videos, but I have a working model.
However, I have no idea which diodes to use, or what resistance and capacitance. I just picked values that I've seen in other circuits.
The green line is input before transformer, the blue line was measured at the capacitor.
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1N4148 isn't what you want there, it is for other use
1N4007 is commonly used
An upgrade can be 1N5408
Using UF diodes instead of 1N diodes will be a further increase
47uF Is a good value, sometime you see also 100uF
(higher values can be used but we are talking of a small SE here)
Franco
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+1
if you look at the schematic, the cathode lists 8.5vdc and 150ohm R
ohmslaw says 8.5vdc/150 = ~57mA
add a fudge factor to say 70mA
do the ohm thing again using 300vdc (plate) 70mA current to come up with a Resistor
tell you're spice that's the power supply "load"
start with 47uF> 470ohm> 47uF see what you get
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Thank you kagliostro and shooter!
I still need to model a correct PT. I'm not getting the correct voltage at B+ but the schematic is running without errors.
I searched and searched for a 1N4007 model and finally found one. I pasted the .model line into the "standard.dio" file and I guess it's working.
Here is a screenshot with updated components and values:
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R1 needs to be in series with the B+ line, NOT to ground, so between the 2 caps +'s
EDIT:
added graphics :laugh:
example of a "typical" PS setup from my last build
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Thanks. Here's a 1:4 ratio PT. Blue line measured at C2.
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> I still need to model a correct PT.
Why stop there? The PT only reflects what it can suck from the power line. Also limited by the utility generator. How is the boiler fire? Is the waterwheel clogged with fish?
"Normally" it is sufficient to MODEL a B+ with a battery, maybe a few ohms in series.
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can you "measure" with spice the AC component on the blue line? as long as you're playing :icon_biggrin:
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Thanks PRR. I know I'm not doing this in the most practical way, but the more I put into it the more I learn. Example: I've gained a rudimentary idea of how inductance ratios relate to winding ratios (fwiw).
shooter, not sure if this is what you wanted to see. I seem to have recorded the db levels of ac frequencies:
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my twisted logic thinks it's telling me >60hz you're AC is down 100db, in my old life down 120db ticked the box for passes spec :icon_biggrin:
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It drops off quite a bit right before 60hz:
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move on :laugh:
your meter and ear will dial it in :icon_biggrin:
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move on :laugh:
your meter and ear will dial it in :icon_biggrin:
Thanks shooter!
In that case it's time to start ordering parts.
After I get a layout worked up - with some help - maybe someone here can walk me through the process of ordering a custom turret board here at Hoffman.
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After I get a layout worked up
:icon_biggrin:
the .DIY board maker software is pretty easy on a 2 or 3 tube'r, 7 tubes, I'm real busy with OTHER things :laugh:
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It looks like I could start with Robinette's "Micro Champ" layout and edit it in DIY Layout Creator, a lot of preliminary work being already done.
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Wow! I thought schematics were challenging to read! Layouts are frying my brain.
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Layouts are frying my brain
:icon_biggrin: If you live in a legal state, it helps :m13
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:laugh: Virginia is getting close, I hear.
OK, some slow progress. Maybe 25% along. I'm sure there are mistakes. :help:
The EF86 has its own "special" pinout that I'm having to learn. Cathode (3) is tied to G3 Suppressor Grid (8).
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The EF86 has its own "special" pinout that I'm having to learn. Cathode (3) is tied to G3 Suppressor Grid (8).
It's acceptable to plagiarize. Here's an EF86 layout that I know works. Maybe it can help with your layout...
http://sluckeyamps.com/dual_lite/dual_lite.pdf
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Thanks sluckey! That's a lot more elegant than what's on the AC4 schematic. My voltage measurements in LTSpice were all wrong anyway. I'm going to "plagiarizer" it!
I had a difficult time trying to layout the parts, anyway, as you can see:
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> My voltage measurements in LTSpice were all wrong anyway.
Between an inexperienced SPICE, and a known-good plan from the Old Days, trust the old info.
I've spent some years trimming SPICE models of triodes for less-wrong results. My pentodes are so far out that I don't even try.
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I can't argue with that PRR! I know my Spice modelling can't be anywhere near correct. Because I really have no idea what I'm doing! But, it's fun trying. Thanks, and it's good to know that the pentode models are suspect. I've tried about three different ones and they all act differently.
So, sluckey, I've put your preamp circuit in my schematic and tomorrow I'll see if I can "translate" it into layout form. I really appreciate the help. I appreciate everyone's help! This is going to be a cool amp.
But, because I can't leave well enough alone I took voltage readings with sluckey's circuit. Everything measured correct except the plate voltage. I changed the 220K resistor to 56K and got the correct voltage.
But, I'm keeping it at 220K for the build. That's the proven real-world value.
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> correct except the plate voltage.
Where is the screen voltage?? That is an important part of bias.
As a rough rule: the happy *ratio* of RP to RS is pretty consistently 1:5 for many small pentodes. You seem to be happy with a very different ratio. However you also have a VERY unusual 6.9k RK where 1k-2k is more normal.
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.
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So, sluckey, I've put your preamp circuit in my schematic
Your plate resistor and cathode resistor are quite different from my circuit. Gonna have a pretty big impact on operation.
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Now I'm not sure what I posted last night.
I don't think I can take voltage reading in LTSpice, until I can figure out how to set up simulations. I'm doing something completely wrong, and the EF86 model is probably wacky, too. [I get 16.8V on the plate, 270V on the screen, and 6.8V on the cathode. :dontknow:]
Anyway, I think this image is correct:
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In many SPICES, "1m" is one milli Ohm.
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In many SPICES, "1m" is one milli Ohm.
Holy ____!
I changed the 1M to 1Meg and I have just under 98v on the screen and about 96v on the plate. The cathode is about 2.2v. That got it way closer to sluckey's schematic.
My brain doesn't see the most basic stuff. That would never have occured to me. Thanks PRR.
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I'll add, that's with 270v on the B+
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I have just under 98v on the screen and about 96v on the plate. The cathode is about 2.2v. That got it way closer to sluckey's schematic.
Good enough. May just be the difference between a real world meter, tube, components, etc and a virtual spice world.
BTW, I had 266v for B+ when I made those measurements.
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Cool. The EL84's measurements seemed closer, too. Plate and screen were 270ish and cathode was about 8.5 - 9v. Your Dual Lite is 317, 295 and 10v respectively.
The whole section of my schematic with the volume and tone pots, and the coupling caps needs work. I'm guess that is having an effect, too.
Thanks sluckey.
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Good enough. May just be the difference between a real world meter, tube, components, etc and a virtual spice world.
Yeah, I played around with it some more. The only way I can get the screen a couple of volts negative of the plate is by adding a 68K resistor in series with the 1meg. At the same time that raised the plate voltage a bit. I think someone in another thread said, "everything effects everything" in tube amps.
Your design works in real life, so it's surely something quirky with my LTSpice model. But, I am learning stuff by watching how resistor values interact, etc. It's all new to me and really interesting.
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May this be of your interest ?
(https://i.imgur.com/8601vrx.gif)
Franco
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Thanks Franco!
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I really don't know what the real-world B+ will be. So, this is all just for fun. But, with the simulation B+ swimming along between 266v and 270v and a 56k resistor in series with the 1meg, I get 99v on the plate and 94v on the screen.
I'll probably do some research on affordable transformers next. It looks like I'll need a 150-0-150 PT and a 5K8ohm OPT. I guess I'll start by looking at Hammond? Let me know if that sounds wrong. Thanks.
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Hey, I thought I'd share my latest partscaster, the reason I'm working on this project.
It's not for everyone, but it's everything I like. Vintage 50's Mexico 7 1/4" neck; Kluson Supreme 18:1 vintage tuners; 4 1/2 lbs. Warmoth Vintage White Hybrid Strat body; Alnico 2 Bootstrap PU's; reverse controls; Tusq XL nut. It's light and I love the Alnico 2's. I just need a decent home amp.
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The Mercury Magnetics are too pricey for me with the current state of things.
The Hammond 269BX is 300V and 6.3V@2A for around $51.
The Hammond 125CSE 8Watt is $50.
It looks like these will work. And, $100 is about what I wanted to spend.
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I'd rather have the 269AX
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I'd rather have the 269AX
Thanks. I see that it's 250V. That will lower my B+ so should I do away with the 470 resistor, or run the amp with lower voltages?
I got the EF86 wired up in DIY Layout Creator. I'm sure there's a cleaner and "electrically better" way to do it!
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Thanks. I see that it's 250V. That will lower my B+ so should I do away with the 470 resistor, or run the amp with lower voltages?
Just stick to the schematic. PT is 250-0-250, dropping resistor is 1K (not 470Ω), and use an EZ80 or EZ81 tube rectifier. Otherwise, your B+ will be around 400V rather than 270V and all that spice time just goes down the toilet.
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Thanks sluckey. That sounds great to me. I appreciate all your help. It's time to focus on the layout.
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The pot wiring needs work, as does the grounding layout. I'll research those things some more. I think everything else is ok.
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Unless I'm missing something, the 269AX voltage at 115 ma. is 125-0-125. The 250 volt rating is across the secondary winding (red to red).
http://www.hammondmfg.com/pdf/EDB269AX.pdf (http://www.hammondmfg.com/pdf/EDB269AX.pdf)
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Unless I'm missing something, the 269AX voltage at 115 ma. is 125-0-125. The 250 volt rating is across the secondary winding (red to red).
Good catch. Huge mistake on my part. Sorry. :embarrassed:
Looks like the 270CAX or 270DAX would be good candidates.
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Thanks 66Strat!
Here's an updated layout:
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There is no center tap on the 6.3 heater winding.
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Thanks sluckey. I'll fix that.
I sort of used the 5F2-A as a reference to try and figure out how to wire the pots. This is an area that confuses me. Another change I made was to use a dedicated power switch instead of the Tone/On-Off pot shown on the original schematic.
I'm off to teleworking. Thanks again for all the help I'm getting here. I think it would be great if this turns out to be a successful amp that others might want to build.
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Not trying to be "that guy" but, the 270CAX wiring diagram shows a center-tap for the 6.3 volt heater winding. The 5 volt heater winding does not have a center tap. I like this power transformer. The 5 volt winding would allow the use of a 5Y3 in lieu of the EZ81 should you choose.
https://www.hammfg.com/files/parts/pdf/270CAX.pdf (https://www.hammfg.com/files/parts/pdf/270CAX.pdf)
Good looking guitar btw. :thumbsup: I've emailed Bootstrap regarding some alnico ii pickups for my Tele.
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Thanks 66Strat.
The Bootstraps are an amazing buy.
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Not trying to be "that guy" but, the 270CAX wiring diagram shows a center-tap for the 6.3 volt heater winding.
Haha. I'm gun shy now. Definitely gonna start looking at the pdfs. Hammond's quick listing did not show a CT.
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Not trying to be "that guy" but, the 270CAX wiring diagram shows a center-tap for the 6.3 volt heater winding.
Haha. I'm gun shy now. Definitely gonna start looking at the pdfs. Hammond's quick listing did not show a CT.
There is definitely a problem in Hammond's documentation. Either the listing is wrong, or the wiring diagram is wrong. I guess you have to buy the transformer to find out what it is. :icon_biggrin: Gee, that has a familiar ring to it. :l2:
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Would it be bad practice to build the vibrato circuit on its own small board? I watched some videos of 1960's AC4's and the vibrato is pretty nice. Maybe I could add it on later after the main amp is tested and working.
I'll probably try to draw a layout for it, too. It's good practice.
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If it were me, I would put it all on one board and use the AA764 Vibrochamp layout as a starting point.
https://el34world.com/charts/Schematics/files/Fender/Fender_vibro_champ_aa764.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_vibro_champ_aa764.pdf)
It wouldn't take much to adapt this layout to your project.
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Thanks 66Strat. My main reason is that this will only be the second tube amp I've ever built, and I need to adhere to my original plan which was to keep it simple and make it work! My first project - that I finished last month - is a Tubelab SET EL34 stereo amp. It works and sounds great. Maybe I just got lucky, but it worked as designed from the first time I powered it up. But, I built the most basic version with no extras and used good parts.
I had nothing more interesting to do, so I went ahead and drew a layout for the vibrato circuit. Whether I use it or not, I got some more practice in Diy Layout Creator.
But, since I've already made a few changes based on sluckey's Dual Lite, it might not drop right in, anyway.
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Well, I just ordered a custom turret board, components, and hardware from Hoffman. He had every component value I needed, and it was a lot easier selecting parts that it is at Mouser, etc.
I'll order the transformers, tubes and a chassis next.
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Tubes and transformers are ordered. I haven't decided on a chassis yet.
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I should be getting most of the parts in the next couple of days, or so.
I solved the chassis dilemma, though it's not pretty. I had an old solid state Fender Frontman Reverb with an 8" speaker that a neighbor gave me. The chassis space in the cabinet is 11.75" x 7" x 2" and Hammond has an 11" x 7" x 2" aluminum one. I figure I can have a local laser engraver make an 11.75" wide faceplate to cover the gaps on either side.
There's not as much to chose from in the 8" speaker realm, and I know very little about guitar speakers. Some people online seem to like the Weber Blue Pup. There's also the Celestion Eight 15. I'd welcome any suggestions or advice on speakers.
Thanks,
David
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If you haven't already ordered your chassis, you might consider having one custom made. I plan on using this guy for a 5Cish5 Pro project.
http://www.juicyamps.com/chassis.html (http://www.juicyamps.com/chassis.html)
I haven't had any experience with him yet, but his products sound promising. I was thinking of polishing the aluminum and hand lettering the face plate with a paint pen. In a former life, I was an architectural draftsman and was pretty good at lettering.
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Thanks JT, I'll take a look at that.
First package of parts arrived. Maybe start some soldering this evening...
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I tried my best to follow these grounding guidelines: https://el34world.com/charts/grounds.htm
Preamp section, pots and input are grounded through input jack connection to chassis.
Power section, speaker jack (and OPT), 6.3 CT, and incoming AC earth ground will connect to PT mounting bolt.
The only thing I'm aware of that I changed is not soldering the bus wire to the backs of the pots.
I'd appreciate some expert eyes on this. I want to start soldering!
Thanks,
David
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I didn't verified the layout, only one council, if I can say
often SE amp has at least a bit of humm problem
to try to minimize it, why don't use a third e-cap on the PS ?
say a ... 33uf or, better, 47uf connected as first cap and followed by a ... 120R till 330R resistor
Franco
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don't need to solder to the back of pots, builder choice thing.
pin 8 of the el84 is not used, you have a jumper to pin 3 :dontknow:
As Franco stated, SE likes to hum if not heavily filtered. My last build I wound up tacking in a couple extra PS caps, but not til I proved it was good working amp.
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Thanks Franco and Shooter!
The two turrets on the far left of the board can easily accommodate an extra filter cap. The ground wire doesn't need to use them (as shown).
shooter, I also jumpered 8 and 3 on the EF86. Should I remove that one, too?
If I could also ask one other question: Does the wiring on my tone and volume pots look OK? I just can't get my head around how that circuit works. Here's the original schematic below.
And, I agree, let me get it running first, and then I'll work on improving it! Thanks, again.
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I also jumpered 8 and 3 on the EF86. Should I remove that one, too?
You must have a jumper between pins 3 and 8. You also need a jumper between pins 2 and 7 that connects to ground.
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Your tone control is not wired correctly.
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Thanks sluckey.
The EF86 wiring diagrams I could find online, and the original schematic, indicate - to my eyes - that 2 and 7 can connect to ground via the cathode bias cap (please see my new layout). I did it that way.
I've studied the schematic some more, and it shows the tone pot Wiper connected to ground. So, I've rewired it that way. :w2:
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A small thing
the yellow wire that joints the 6.8K resistor to pin #2 of the Power Tube isn't placed correctly
Franco
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The Hammond chassis is an almost perfect fit. There's just a small gap on either side that a faceplate will cover up.
I also made a few changes to my layout:
1.) I put the power switch before the fuse so that no one can get shocked by changing a good fuse if the amp is off, but still plugged in (of course, someone could still try to change a fuse with the power switch on, but that's a symptom of a bigger problem).
2.) I moved the grid stoppers to the tube pins as I learned that the closer they are to the grids the better they work.
I'm just waiting on a few more parts to arrive. I bought the wrong socket for the EL84; I bought the kind with the shield and the tube is too tight. (Hoffman explains all that in his parts store, but I failed to read it!)
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A small thing
the yellow wire that joints the 6.8K resistor to pin #2 of the Power Tube isn't placed correctly
Franco
Thank you Franco. Please tell me how it should be connected.
David
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In the image I see it connected near the pin #3 (also if not exactly to pin #3)
Sorry, you only miss pin #2 on the socket layout
Franco
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Thanks! Just sloppy drafting on my part. Here's an updated layout:
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Would there be any potential noise reduction benefit in connecting the EF86 shield directly to ground, rather than elevated at cathode potential?
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66Strat, I found this thread:
https://el34world.com/Forum/index.php?topic=432.msg454#msg454
After your post I was thinking the socket's mounting bolt would be good. I don't know what's best.
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I have neither a great knowledge of electronics, nor a valuable experience in the guitar world; but I've been messing around the schematic of the AC4 for two builds now, so I felt like I had to share some 5/6 thoughts - below - for what they're worth... :occasion14:
Firstly, I haven't checked the schematic in great detail but the 5.0V mention close to the HT leads to the EZ81 is pretty confusing, especially when dealing with people used to work with US rectifiers - as a starter.
Secondly, your schematic appears to lack filtering. In my archives, the original schematic shows two 33uF then a 8(10) uF capacitor and the EF86 is NOT connected to the B+, but to some C+ sharing the HT node of the third cap.
Thirdly, there are a few boards layout for the amp that have been posted on a few forums so you should be able to see them and compare with yours.
Fourthly, if you want the "vintage tone" you could consider a smaller OT. The originals used to show a ridiculously small one and - if you're in the US, it seems that Mercury Magnetics have a replacement part, with ref. #VXP-4. I won't be surprised that they also have a PT too. It just depends on what you're after: a replica or some modded version.
Fifthly (this word sounds pretty unusual, doesn't he?), I feel like the PT with its 5V windings is overkill. See the fourth point above. NB: I might be wrong!
Finally, I decided to build one using the point to point layout of the late sixties - the one with a ground bus - mostly, AND with the tremolo, because I felt the effect was worth the little extra effort. I started wiring the heaters then the tremolo. That way I had no temptation to forget it in the end... :icon_biggrin:
Might https://ampgarage.com/forum/viewtopic.php?f=10&t=33245 be of some interest to you?
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Firstly, I haven't checked the schematic in great detail but the 5.0V mention close to the HT leads to the EZ81 is pretty confusing, especially when dealing with people used to work with US rectifiers - as a starter.
Oops! Thanks ChopSauce. Since I had never used or heard of Diy Layout Creator before this project I stole an existing file from Rob Robinette's site and used it as a template. The "5.0V" text was left over from his layout, and I missed it. Thanks for spotting that; I deleted that text.
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Nice! Did you build that cabinet? Looks great.
From your build: "Mais qu'est-ce que vous allez faire avec le chocolat?" - I was also wondering how you point-to-point wire a candy bar. :icon_biggrin:
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Concerning filtering: Is there a maximum size capacitor that the EZ81 can safely see in front of it? The datasheet below indicates a limit of 50uf - see where I outlined it in red. Am I reading that correctly? Does that only apply to the first capacitor in the filtering chain? So, if I install an additional 47uf in front of the two 22uf capacitors is that within the tube's specs?
Thanks,
David
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that's how I read it. You have a 47uF + 22uf for a total of ~70uF, you might
get away with that. keep an eye on the rectifier when powering up to see if it gets sparky inside
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To be safe, why not move the 1K dropping resistor over between the 47Uf capacitor and the first 22Uf capacitor? The difference in overall filtration would be negligible and would satisfy the rectifier maximum capacitance restraint.
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Great minds think alike! Actually, I was considering two 500ohm's, but that's just one extra part that could fail down the road.
Thanks.
The last of my orders arrived today. I now have all of the parts except grill cloth for the cabinet. I'll deal with that later. I've been too busy with telework this week to get started, but tomorrow I'll begin putting this thing together.
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Yes, I made the woodworking myself. I read http://www.geofex.com/Article_Folders/voxamp/voxcab.htm then decided to try double dowels. Afterwards I'm not sure that was that much worthy but I do wanted a TV front cab and there was no such thing readily available.
So you already ordered parts? One thing you can try, to improve filtering is soldering the two 22 uF caps in parallel. That would make for a slightly big 44uf cap but still it is manageable - for example (I did it) by gluing the second 22uF on the first and the second one for the B+, which could be a 33 and then a 10uF, to match the original schematic. Maybe?
Since you should be able to find the dimensions of the caps, it is fairly easy to prepare everything until some possibly last order?
It's up to you, but you should at least keep a place for a third cap on the board - even if you don't wire it at once.
NB: since I'm here, I feel like asking because the question might arise soon - if I can judge by you don't seem to refrain modding the schematic...
A common mod. to the AC4 is lowering the R8 resistor of the EF86 screen grid - see https://el34world.com/Forum/index.php?action=dlattach;topic=25921.0;attach=83322;image - to a few MOhms value, closer to the AC10 or AC15 ones.
I have been willing to do the maths as per Merlin Blencowe's http://www.valvewizard.co.uk/pentode.html insights but Merlin states that the screen current is fixed by the datasheet (that is construction?) and that the screen grid resistor should be set accordingly...
So, what happens when the screen grid resistor departs (much) from the corresponding value :w2:
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Some suggestions:
- Swap the positions of the 47Uf capacitor and the first 22Uf capacitor. Connect pin 3 of the EZ81 to the 47Uf capacitor. Connect the 47Uf capacitor to the 22Uf capacitor to the left with a 1K ohm resistor. This is the B+ supply for the EL84.
- A 10K to 22K ohm resistor connects the 47UF capacitor to the 22 UF capacitor to the right. This becomes the EF86 B+ feed.
- Split the filter grounds so that the filtration for the EL84 is grounded near the power transformer and filtration for the EF86 is located near the input.
- Connect pin 7 of the EF86 to pin 2 and connect pin 2 directly to ground. I showed this as connecting to the input ground, but it could/(should?) have its own ground.
See attached.
Edited to delete duplicate verbiage
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Splitting the grounds does make sense, but I don't understand the rationales for putting a 10 to 22k resistor between the first two caps - instead of the 1k from the original schematic?
Maybe the layout missing one filter cap (for the EF86) - if I'm not mistaking - is the reason for some misunderstanding between us?
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About the added cap and the resistor
Post #82
(https://i.imgur.com/sgimJD9.jpg)
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if you connect the e-cap that feeds the preamp to the first cap node instead to the second node you lost smoothing effect
BTW the original AC4 uses 2 x 32uF capacitors on the PS, why you lowered the value to 22uF ?
OK you added a 47uF but your interest is to increase the filtering to avoid humm, don't you think it will be better to add capacity without decrease it in the further node ?
Franco
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Thanks Franco. I'm not sure now why I switched to the 22's. I think I got that off of Sluckey's Dual-Lite schematic when he was helping me. :dontknow:
I will build it with the parts I have and in the mean time, I can buy some more caps and resistors.
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Don't worry. Building this with the parts you already have is what everyone is suggesting. Now it's up to you to choose between 66Strat suggestion for 47, 22 and 22uF, respectively or mine for 44, 33 and 10, which is closer to the original schematic but less usual even though stacking caps (the two 22uF in parallel side by side with the third 33) also shows on renowed amps.
Certainly the availabilities of your favourite amp shop will make the decision for you, but at least you have some choice.
Maybe you should print out the schematic and always keep it with you. That's what I did with my first build, at last.
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Splitting the grounds does make sense, but I don't understand the rationales for putting a 10 to 22k resistor between the first two caps - instead of the 1k from the original schematic?
Maybe the layout missing one filter cap (for the EF86) - if I'm not mistaking - is the reason for some misunderstanding between us?
The rationale is to provide two separate B+ nodes, one for the EL84, and one for the EF86. The EF86 draws about 1/10th the current of the EL84. This allows the use of a larger dropping resistor to better separate the B+ nodes.
About the added cap and the resistor
Post #82
(https://i.imgur.com/sgimJD9.jpg)
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if you connect the e-cap that feeds the preamp to the first cap node instead to the second node you lost smoothing effect
BTW the original AC4 uses 2 x 32uF capacitors on the PS, why you lowered the value to 22uF ?
OK you added a 47uF but your interest is to increase the filtering to avoid humm, don't you think it will be better to add capacity without decrease it in the further node ?
Franco
I was rearranging the components that dwinstonwood selected. The 47uF filter capacitor at the input from the EZ81 provides the initial filtration for both EL84 and the EF86. I believe that the ripple at this point can be calculated as Vpp=I/(2 x C x F), (that's if I got the right formula).:laugh: I at this point is the combined current of both the EL84 and the EF86.
Both B+ nodes for the EL84 and the EF86 are filtered further by the 22 uF capacitors at each of these nodes.
See attached.
Edited for grammar and verbiage omissions.
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Thanks everyone! I will have to take some time to sit down and digest the suggestions. My immediate response is that it's over my head. But, once I study what's going on it will make sense. I have to "walk" through schematics very slowly before they appear legible!
One thing I can do an acceptable job at is working on the chassis. I drilled it to death with a step bit and I've installed all of the hardware (except the power cord strain relief). I'll drill the mounting holes for the turret board after I populate it and decide the best location for it in the chassis. I will eventually mount the EF86 socket with rubber grommets; I understand it's a temperamental thing.
I really do appreciate all of the help and interest in this project. I feel like I've already learned a heck of a lot more than I would have building a "paint-by-number" kit.
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OK, this arrangement will bw similar to the VOX AC30 PS (each node is feed directly from the rectifier)
(https://el34world.com/Forum/index.php?action=dlattach;topic=25921.0;attach=83992;image)
But this way you have less ripple smoothing (you don't benefit from previous filters like in many other amp PS arrangement)
To me the correct way to arrange your PS will be this
(https://i.imgur.com/kj9k6Uh.jpg)
(EF86 filter ground can be connected to the ground of the input jack, remember this must be a not isolated one, it will be easy to verify if this is the better way or if your way will be better connecting it to the PT CT Ground Bolt with the other filter)
Franco
p.s.: Maybe (as you have a missing 12AX7 tube in the circuit and so a bit less consumption, you have to increase a bit the value of 1K resistor on the PS, or maybe both resistors of the PS has to be increased a bit; when you finished the amp verify that voltages are within + or - 10-20% of the project - schematic say 270V at EF86 node)
Franco
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Splitting the grounds does make sense, but I don't understand the rationales for putting a 10 to 22k resistor between the first two caps - instead of the 1k from the original schematic?
Maybe the layout missing one filter cap (for the EF86) - if I'm not mistaking - is the reason for some misunderstanding between us?
The rationale is to provide two separate B+ nodes, one for the EL84, and one for the EF86. The EF86 draws about 1/10th the current of the EL84. This allows the use of a larger dropping resistor to better separate the B+ nodes.
This is what the original schematic provides and I recommend that it is reproduced as closely as possible so I definitely think we're currently discussing off grounds - what I refered to as "some misunderstanding"... :dontknow:
Not having the exact same components is not even a reason for not managing to adhere as closely as possible to the original schematic, according to my informations:
- stacking resistors (example given: two 500R to reach a 1k value) is no trouble at all for Mr Howard "Alexander" Dumble, in the same way as stacking caps was no trouble for Mr Ken Fischer so I definitely don't see the reason for all these deviations...
Even the kind suggestion to increase the value of the first resistor, due to the missing 12AX7 is based upon the assumption that the power transformer is the same, which is unlikely.
Anybody does like he wants. For my own, I started after dwinstonwood and my build is already finished - but the two 100R resistors I have been waiting for over two weeks now. I just sticked to the original schematic (and to gutshots of the real thing.)
For my first build, I was full of doubt and managed to follow kind advices from a lot of people at the same time. A few years latter I have revised the ciruit to be closer to the standards.
So enjoy the build and then, it will be time for mods or any kind of experiments, possibly... :occasion14:
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OK, thanks everyone.
So, here's what I'm going to do... for the initial startup and testing I'm going to use what I have: EZ81 > 47u > 1KR > 22u > EF86 and OPT, basically following the wiring of the schematic.
F&T seems to make 30u's, I'm having trouble finding 32u axial caps. Unless someone knows of a source I'll go with the 30u's (my other caps are F&T). Then, if it's just too noisy I will rewire the split filtering setup using two the grounding points.
On the mechanical front, the chassis is a good fit in the old Fender Frontman cabinet. I shifted the chassis over to the right so that there's as much room between the PT and the speaker frame as possible to allow plenty of clearance for different speakers.
Once I add a custom faceplate and some decent grill cloth it will not look as ugly as it does now. :laugh:
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@ ChopSauce
May be I don't understand exactly what you are saying, due to translation
The reference schematic used for this build is this
(https://el34world.com/Forum/index.php?action=dlattach;topic=25921.0;attach=83324;image)
and it is derived from this
(https://el34world.com/Forum/index.php?action=dlattach;topic=25921.0;attach=83322;image)
he original PS shown uses two capacitors (32uF) + one resistor (1K - 5W)
there is only one node that is connected to the circuit, as you can see, B+ is derived from the +270V point and feeds the whole circuit, the two 12AX7 tubes and the EL84 tube
no distinction on B+ node on this version of the AC4 schematic, for the preamp tube and the power tube only one source
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The use of an added e-capacitor (with resistor to form an add on filter node) is often seen in amps like champ 5F1 that has the tendency to humm a bit more than what we would like
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The use of a separated ground at the input jack is "good practice" applied in many amps; I told to use it and if on trouble, the preamp ground can easily be connect to the power tube amp ground (at the CT Bolt on chassis), to verify if this give on this circuit/layout a better performance
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I didn't told the value of resistor/resistors on PS must be increased, I told maybe, NOT it must (read what I've write on previously post), according to voltages within + or - 10-20% on the circuit (that are fine); of course, the closer to what is write on the schematic, the better
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@ dwinstonwood
About 30uF or 32uF e-cap values ............. use what you can easily find with the nearest values, remember that e-caps that are NEW can have a tollerance till + or - 40% of the value with which they are labeled (some e-cap also arrive to have a 50% tollerance)
Franco
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About 30uF or 32uF e-cap values ............. use what you can easily find with the nearest values, remember that e-caps that are NEW can have a tollerance till + or - 40% of the value with which they are labeled (some e-cap also arrive to have a 50% tollerance)
Franco
Thanks. Is there a real advantage to using two 32u caps vs. one 47u and one 22u cap? With the manufacturing tolerances would the 5u difference even matter?
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Is there a real advantage to using two 32u caps vs. one 47u and one 22u cap?
Depends on how the circuit is arranged
The principal difference I can see, is that your tube rectifier will be a bit less stressed using a 32uF cap as first filter cap
however 47uF is within the EZ80 spec (50uF)
http://tdsl.duncanamps.com/pdf/ez80.pdf (http://tdsl.duncanamps.com/pdf/ez80.pdf)
I'm not a simulation program fan, however I think you can play a bit with Duncan PSU designer and see what will happen
http://www.duncanamps.com/psud2/ (http://www.duncanamps.com/psud2/)
With the manufacturing tolerances would the 5u difference even matter?
No
Franco
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There is only a few mv difference between any of the configurations. See attached.
I backed into the EZ81 supply voltage based upon an estimated 57ma total current draw of the AC4 circuit including the tremolo (54ma for the El84 + 1ma for the EF86 + 2ma for 2 AX7 triodes). The 57 ma current draw across the 1K resistor results in a 57 volt drop. Adding the 57 volts to the filtered 270 volts indicated in the schematic equals the estimated supply voltage of 327 volts.
I then calculated voltage drop and ripple based upon the current of each tube in the circuit without the tremolo. VPP at the EZ81 is calculated as VPP = I/(2 x f x C).
I used the RC Ripple Attenuation calculator at Ambooks. com to calculate the attenuation in dbs of the RC filters.
https://www.ampbooks.com/mobile/amplifier-calculators/RC-ripple-filter/calculator/ (https://www.ampbooks.com/mobile/amplifier-calculators/RC-ripple-filter/calculator/)
I then used db to dbV calculator at Ambooks.com to convert dbs to volts.
https://www.ampbooks.com/mobile/amplifier-calculators/dB/calculator/ (https://www.ampbooks.com/mobile/amplifier-calculators/dB/calculator/)
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and it is derived from this
So why is an important filter cap left out of the derived circuit??
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I missed it too. :embarrassed: :laugh:
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So why is an important filter cap left out of the derived circuit??
:laugh: :laugh:
Seems that the PS is to be updated
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Someone gived the advice but seems that no one payed attention (or, at least I didn't :icon_biggrin: )
Secondly, your schematic appears to lack filtering. In my archives, the original schematic shows two 33uF then a 8(10) uF capacitor and the EF86 is NOT connected to the B+, but to some C+ sharing the HT node of the third cap.
Franco
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Wow...
So here is the "original" schematic. The only difference is the 47u and 22u instead of two 32u filter caps.
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This also gives me voltages in LTSpice similar to those shown on the original schematic: 264V B+ and 205V plate. The schematic shows 270V B+ and 215v plate. This slight difference is probably due to my EZ81 model, etc.
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I just caught up n see I'm 2 blame :think1:
when I cut up the schematic in post 2, I took too much tremolo out :BangHead:
watch the watchers :icon_biggrin:
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I chalk it up to target fixation.
I was focused on the two 32uF capacitors in the power supply and missed the 8uF cap.
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No problem guys! I'm learning from all of this. I'm just glad I didn't solder every up today like I had planned to!
OK, I tried to redraw my layout to accommodate the parts in the original schematic: the 22k resistor and the 8uf electrolytic.
I'm not sure if the 8uf cap - the one that was missing - is wired correctly. I tried to follow the schematic, maybe too literally.
Please scrutinize...
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I'm not sure if the 8uf cap - the one that was missing - is wired correctly.
It's not.
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Maybe this is closer? :dontknow:
At this point my options are to buy some turrets, re-drill my board and try to install them, or just order a new board. Hard to believe a single ended 3-tube amp with two knobs would end up being this complicated. But, I have to much invested to not finish it.
David
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:thumbsup:
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Maybe this is closer? :dontknow:
At this point my options are to buy some turrets, re-drill my board and try to install them, or just order a new board. Hard to believe a single ended 3-tube amp with two knobs would end up being this complicated. But, I have to much invested to not finish it.
David
except make use of a 100K for 33K shown that connects to EF86 pin 9 - you'll want the 100K for the pentode, 33K is fine for triode, however pentode wants larger grid stopper to block RF.
--pete
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...
--pete
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Don't forget the pre-amp filter cap
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Thanks sluckey, I take the thumbs up to mean it’s right.
Pete, thanks, I’ll use a 100k. Thanks for fixing the heater wires. Another leftover from the layout I used as a template.
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Don't forget the pre-amp filter cap
The one I have circled has the lowest VPP ripple for the EF86. I assume this is the best one to use?
Thanks 66Strat for crunching those number!
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Don't forget the pre-amp filter cap
Looking to the original schematic, this is what I see
(https://i.imgur.com/E5vTLh9.jpg)
(assignation to ground are supposed, other things are as per schematic)
(https://el34world.com/Forum/index.php?action=dlattach;topic=25921.0;attach=83322;image)
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May be you can re arrange the use of the turrets for the added filter node to place the 8uF, renouncing to place the 8uF cap near the EF86, this way you can use the board you have
(https://i.imgur.com/Omwbwbe.jpg)
Franco
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Maybe this is closer? :dontknow:
At this point my options are to buy some turrets, re-drill my board and try to install them, or just order a new board. Hard to believe a single ended 3-tube amp with two knobs would end up being this complicated. But, I have to much invested to not finish it.
David
What does your existing board look like? What filter caps do you have in hand? My thought is that rather than buying more parts, and re-drilling or buying a new board, it would be more cost effective to move around a few board connector wires. I hate to spend money when it's not necessary.
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Franko, your latest post (Reply #133) would allow me to use the board as it is without turret modifications. Thanks!
What I still have is what I'll call "Conceptual Blindness." I can follow a schematic in the literal sense - the way it was actually drafted, and that's what I drew up in my last layout (Reply #126): the schematic (graphic/abstract) order of the parts.
But... the big step with electronics - a skill that all of you grasped long ago and now seems obvious - is getting to the level where a graphic arrangement of components makes sense as a functioning circuit. This isn't immediately apparent to someone new to electronics, like me.
By studying kagliostro's layout I can see that "electronically" it does what my last layout does, but in a much more logical or efficient (conceptually accurate) way.
In both layouts voltage leaves the node after the 1K resistor and the second 30uf filter cap and passes through the 22k resistor. From this point it encounters the 8uf filter cap. And, from there the voltages branch off to pins 1 and 6 of the EF86 via the 5.6Meg and 220K resistors, respectively.
Sorry for all of that verbiage! Writing it out helps me see it better.
The first tube amp that I built, a few months ago, is based on a PCB. The only wiring involved was hooking up the tx's, inputs and outputs. It didn't push me to learn too much about circuits (other than selecting the best cathode resistors for proper plate dissipation). With a lot of luck and great care, it worked as designed from the first startup. So, I might have gotten a false sense of confidence from that project. I attached a photo of it.
The good news is that I think we're close now to the best layout.
66Strat, I've already ordered more parts. So, I'll just wait a few more days for them to arrive. What I can do now, while I'm waiting, is wire up the chassis hardware: tube sockets, pots, power cord, etc.
Thanks everyone!
David
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By studying kagliostro's layout I can see that "electronically" it does what my last layout does, but in a much more logical or efficient (conceptually accurate) way.
Ahem ... really ... that isn't my layout, it is 66Strat Layout suggestion (Thanks to 66Strat for sharing)
I only rearranged the PS a bit according to the original schematic
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By studying kagliostro's layout I can see that "electronically" it does what my last layout does, but in a much more logical or efficient (conceptually accurate) way.
Well .... no, the best practice will be to have the e-cap that feeds a node as close as possible to the node, in many amp this is disattended and in your case is a way to use the existing board
(Oh, forgot, consider that the layout that show the 8uF placed near the tube, show a very small one, may be you can find one bigger and be in trouble, remember this in your next work)
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If I can suggest
Don't connect the yellow wires to the turrets (one is from the pot wiper to pin #2, via the 6.8K resistor and the other is from the Tip of the input jack to pin #9, via the 33K resistor), go from the wiper and the Tip connection directly to the resistors (good practice is to cover almost completely the resistor and junction wire/resistor with shrink pipe to enforce and protect/insulate the junction)
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If you want to read something of interesting about ground and e-cap placement
http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
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BTW, seems you have a comfortable size chassis ..... have you enough space to place this Board in a way that allow you to add one other small one with the vibrato circuit in the future if you decide to add it ?
Franco
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Thanks, and thanks to 66Strat, too. I've lost track of everyone's contributions.
Also, in previous layouts I forgot to show the PT center tap! I've added it to the one attached.
For signal wire I'm using this: https://www.tubedepot.com/products/mogami-w2330-interconnect-wire
I'll only connect/ground one end of the shielding.
By not using the turrets for the yellow wires that frees up two sets of turrets. I can use these for the cathode resistors. If I need to play with those resistor values it will be easier to swap them that way.
It would be tight fitting another turret board in for the vibrato. It could be done with tag strips, I guess.
OK, here's layout #641:
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The ground of the cathode resistor and bypass resistor of the power tube (EL84) is not connected correctly, move the connection to the 30uF cap (must not be connected to the 8uF cap) (OK, this has been changed and now the layout respects the intention to have a separate ground connection for the preamplifier)
The Heater CT can be connected to the EL84 cathode (it was good practice on some old amps, to have the benefit of a DC elevated Heater CT)
About the board for the tremolo ... we will discuss one other time, but to shrink the relative layout is not impossible
Franco
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Fixed image in my last post.
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If you encounter Humm problem, here a variant to be tried (two, but electrically are the same thing)
(https://i.imgur.com/ccL8NWi.jpg)
(https://i.imgur.com/pXxHTuZ.jpg)
Of course also an increase to 47uF of the first filter cap can be applied
Franco
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A mid-60's example (looks like new caps):
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I've started wiring the chassis. I still need to wire the input jack and pots with shielded wire. I'll try to get that done tomorrow.
I have a question, though. I'm measuring 0.4 Ohms from the speaker jack tip to ground. Is this because a circuit is being completed through the OPT secondary wires? I'm sure I have the speaker jack wired correctly. I'm guessing this is normal.
Thanks,
David
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I'm measuring 0.4 Ohms from the speaker jack tip to ground. Is this because a circuit is being completed through the OPT secondary wires?
yes. All good.
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> completed through the OPT secondary
Ideally the DC resistance of an audio winding is dead zero.
Ideal transformers are never in stock. We accept DC resistance of 5%-10% of the intended load resistance.
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Thanks sluckey.
Thanks PRR, 5% of 8 Ohms is...
0.4 Ohms. Cool.
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I've been reading up on grid stopper and grid leak resistors. I have a basic understanding of what they do and why they're used.
My question is: should my EF86 100K grid resistor go straight from pin 9 to ground, or can it be in series with the 1M resistor and be grounded at the input jack as seen in my current layout?
Looking at Hoffman's 5F2A layout, it looks similar to what's in my layout. There's a 33K grid resistor attached to pin 2 of the 12ax7 and a wire that runs to the input jack where the 1M resistor connects to ground.
Thanks,
David
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grid stopper - in series of input signal to grid (your 100K) placement is usually between the grid and grid leak resistor.
grid leak - grid to ground or negative voltage reference (your 1M).
--pete
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Thanks Pete!
I think this is way I originally had it. :w2:
Here, the 100K grid stopper is connected directly to pin 9; the 1M grid leak is connected to ground via the input jack.
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What's confusing me is the original schematic that "seems" to have the resistors the other way around. :w2:
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Quit torturing yourself. You have the schematic. Just build it.
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Quit torturing yourself. You have the schematic. Just build it.
Aye aye!
It is now brilliantly clear why people solder their pots in external jigs.
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Making progress. The Hoffman turret board was a pleasure to solder up.
Next session will be connecting everything up (the right way).
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I have one wire left to hook up.
I'm stumped. :dontknow:
Both of my OPT wires are connected to the EL84 - pins 9 and 7. So... how do you all connect your B+ to the OPT?
I feel like it might be so obvious that I can't see it. Can the wire from my second filter cap go straight to the tube? If so, which of the two pins?
Help and thanks!
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Connect the B+ wire to pin 9 exactly as shown on the schematic.
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Thank you sluckey!
It doesn't look like it, but, I am getting somewhat better at reading schematics. Slowly. Studying an old copy of Weber's desktop reference.
OK, I lied. I have one other wire left. I didn't have any 100K resistors (could swear I ordered some). Always one part short.
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OK, I lied. I have one other wire left.
The schematic can solve that puzzle too. :wink:
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I meant I was going to solder the 100K resistor to the EF86's pin 9, and solder the input wire to that.
One more shot of the wiring. It's not art, but it's not too cluttered, either. :icon_biggrin:
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Dang! I left off one connect! I just noticed it in the photo.
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Dang! I left off one connect! I just noticed it in the photo.
Better look closely. I see at least three things on the board that will prevent this amp from working.
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Thank you. Some wire are under the board:
1.) I connected the el84 cathode resistor to the cap (that's what I noticed earlier).
2.) the el84 cathode cap and resistor are connected to the first two filter caps' ground side by a wire under the board.
3.) those four components are connected to chassis ground by a wire under the board.
4.) the third filter cap b+ (for preamp) is connected to the junction of the two angled resistors by an under-board wire.
5.) there's also an under-board wire connecting the .1u coupling cap to the .001 cap connected to the tone pot.
I have it wired exactly like this revised layout.
David
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Some wire are under the board:
That covers the issues I saw.
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>> Some wire are under the board:
> That covers the issues I saw.
We often see (or don't-see) underboard troubles.
Maybe we want clear board-stuff?
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Maybe we want clear board-stuff?
Totally agree. This board is so simple I would have put every jumper wire on top in full view.
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Thanks PRR and Sluckey!
That's a very easy mod to do. I'll make the changes while I'm waiting on the last of the parts.
Looking at the (almost) completed chassis I'm amazed that this project started by studying an 8" x 5" reprint of a 60 yr-old schematic in a book I dug out of the closet.
I definitely came to the right place to attempt this. DIY communities are incredible.
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Moving the wires didn't take long. As you all said, everything can be seen, and worked on without having to remove the board. The one thing you can't see is the chassis ground which is beneath the upper-right corner of the board.
The only thing I know of that's missing is the 100K resistor on pin 9 of the EF86. It's in the mail.
Thanks
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My 2c
I'll put the wires under the board and draw connections on the top of the board as to see what Is connected to what
Franco
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Thanks Franco. I wouldn't have thought of that.
One of many things I'll do differently on the next build will be to orient the tube sockets to make wiring easier. I just lined them all up straight. But, now I realize why rotating them 45 degrees - as I've seen - makes sense.
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The only thing I know of that's missing is the 100K resistor on pin 9 of the EF86. It's in the mail.
100K is not a magical tone component nor is the value critical. I would use anything between 33K and 100K. I used 33K on all my EF86 preamps. Vox used 68K, 100K, and even 220K on the various AC-XX models.
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I wouldn't have thought of that
Here on the forum someone dit it in a much better way (Using a CNC - I don't remember who was)
On that layout there were no hidden connections, but the principle is the same
draw components like in many PCBs, so, why not the hidden connections ?
(https://i.imgur.com/CNUBx2V.jpg)
(https://i.imgur.com/OhBlzfF.jpg)
Franco
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Cool Franco, that's like a high-end kit. What happens when you laser cut turret board material? Would it just burn or melt it? I have access to a laser cutter/engraver at work.
Thanks sluckey. The way I understand it, the grid stopper functions with the tube capacitance to create a low-pass filter. I guess what's important is cutting off the higher frequencies that can cause oscillation?
I tried to look up the capacitance of the EF86, but found numbers like 3.8pf and 5.1pf. I thought tubes had a capacitance of around 100-ish. Otherwise, I was going to use one of those online filter calculators to find out my frequency cutoff point.
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... I have access to a laser cutter/engraver at work. ...
That job was done with a milling CNC router, I've no idea what will happen using a laser engraver on Boards materials
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... I guess what's important is cutting off the higher frequencies that can cause oscillation? ...
To avoid oscillation and to avoid to receive Radio Frequency signals like if your amp is a radio receiver
Franco
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It's time for our history lesson. More than you care to know about the JMI AC4:
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sluckey, so this is what I ended up with, which is what you suggested from the outset!
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*First Power-Up - No Tubes*
Pin 1 of EZ81 socket - 266.2V AC
Pin 7 of EZ81 socket - 266.4V AC
Pilot light glows.
Pin 4 of EF86 (far end of the filament wiring) - 3.376V AC
Pin 5 of EF86 (far end of the filament wiring) - 3.377V AC
Am I ready to install the rectifier and charge the filter caps? I mean, those voltages look good to me, but what do I know?
I do not own a variac. I've read that I should use a low-wattage bulb in my dim-bulb-tester for the initial charging of the filter caps. Is that wise?
Thanks.
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Do you have an alternative cabinet yet?
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Do you have an alternative cabinet yet?
I'm still going to use the Frontman Reverb cabinet for now. I did buy a new speaker and some grill cloth. I'm making a lauan baffle. If this amp ends up sounding good I will most likely add the vibrato circuit and invest in a worthy cabinet. This all started as a "low-budget-stay-at-home" kind of project.
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If you want to check again read resistance to ground in the high voltage path and get several hundred kOhms anywhere and fire her up.
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Than ks Williamblake, but can you elaborate a bit? I should take an Ohm reading from pin 1 and from pin 7 to ground and look for extremely high resistance? That's what I want to see?
Thanks.
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Drain the filter caps and then measure the resistance across each filter cap. Do not want any low resistance readings.
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I have not powered up with the rectifier installed yet. It's too late to get into it tonight. I'll test in the morning.
However, when I try to measure resistance across the filter caps my meter goes to O.F, overflow, so the resistance is higher that what my meter can read; when switched to continuity they read "Open." That must mean they are functioning like capacitors, yes?
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*Second Power-Up - No Tubes*
Installed the EZ81:
329.1V DC at first filter cap.
327.8V DC at second filter cap.
Upon power up the dim bulb lit up and then immediately dimmed.
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Plug it straight into the wall and get your guitar. Time to play.
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*Third Power-Up - All Tubes and Speaker - Weird*
OK, first the good. No smoke, no smells, no humming or noises.
The weird (bad?). Pins 7 and 9 of the EL84 initially rise up to about 258V and then gradually fall and settle at around 135V.
Pin 6 of the EF86 initially rises up to about 241V and then gradually falls and settles at about 112V.
But... I suspect that I destroyed my EF86. Being nervous, I accidentally installed it in the rectifier socket for the first power up. Nothing smoked or popped, but do you think I destroyed it? Maybe something shorted and its not passing current. :BangHead: :BangHead: :BangHead:
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Did you plug the amp straight into the wall?
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Pins one and seven of the ez81 tube socket are the anode connections. Pins one and seven of the ef86 tube socket are respectively the screen grid and internal screen connections. Pins two and seven of the ef86 socket are connected to ground. By putting the ef86 in the ez81 socket, AC from the power transformer was shunted to ground through the pin seven connection.
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Yes sluckey
And it's AWESOME! :m7
The part you won't believe: with the volume dimed and my guitar wide open all I can hear is a very, very low level hum; the transformers, I guess. I have to put my ear next to the amp to hear it.
Volume and tone knobs work great. With the amp cranked all the way I get nice, smooth distortion. At half volume it is still clear and clean. With tone at noon it is warm and full sounding. It has a very usable range, and is not thin or too bright. I get good bass considering the 8" speaker. All frequencies seem to be coming through.
I honestly think it sounds great. I need some time to come back down! My heart rate was pretty high.
I'll post again to personally thank everyone here who contributed. But, a big thanks to you sluckey.
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Of course, all is not perfect. The tone pot works backwards, and it's a bit scratchy. I noticed it makes a rubbing sound when turned (with the amp off). I hope that's not a symptom of a bigger problem, elsewhere.
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The tone pot works backwards
Easy fix. Refer to your most recent layout drawing. Remove all wires from the pot, including jumper between lugs 1 and 2. Now reconnect ground wire to lug 2 only (no jumper needed). And reconnect yellow wire to lug 1. That's all.
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Fixed. Thanks!
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it makes a rubbing sound when turned (with the amp off).
probably knob binding on chassis
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it makes a rubbing sound when turned (with the amp off).
probably knob binding on chassis
Thanks shooter, it actually does this with the knob out of the chassis. It makes a squeaking sound. I wonder if some liquid flux ran down onto the wiper. I can give DeoxIT a try.
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I've been playing the amp for about an hour and it seems stable, nothing has changed in the way it operates. In fact, the transformers are at like room temperature to the touch. The tubes have a nice orange glow that looks normal.
What's still bugging me is what I was doing wrong when I took voltage readings this morning. Why would the B+ voltages rise up and then fall back down? Maybe I was measuring at the wrong points in the amp. I don't know. :dontknow: But, the amp is clearly operating fine. It's plenty loud, and it doesn't overdrive until I crank it.
Anyway, I would like to take cathode bias measurements and calculate plate dissipation, etc.
I'll try measuring again in the morning, because this is going to bug me. Any ideas are welcome.
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Let the amp warm up for 5 minutes.
Measure voltage on the positive end of each of the 3 filter caps on board. Black probe on chassis, red probe on the caps. What have you?
Now measure voltage on V2 pins 7 and 3. Black probe on chassis, red probe directly on socket pins. What have you?
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thanks again sluckey (for your patience). I didn't let the amp warm up this morning. I'll report back tomorrow.
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OK, I took some measurements after the amp warmed up.
Assuming my math is correct, dissipation is a bit high for the EL84 at 12.231 watts. The tube is rated for 12 watts.
First, the filter cap voltages:
1 - 304.5V
2 - 252.2V
3 - 233.6V
Now, the EL84:
EL84 cathode pin 3 - 7.77V
EL84 plate pin 7 - 243.9V
Current draw - 51.8MA
Dissipation - 12.231 watts
If I change the 150 Ohm cathode resistor for a 180 Ohm, that will give me 10.19 watts. That's about 85% or so.
A 220 Ohm would drop me all the way down to 8.34 watts.
What do you suggest? Leave it running over 100% or bring it down some?
Thanks,
David
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I'd leave it alone. 84s are cheap and plentiful.
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Except mine's a NOS Mullard...
Haha, just kidding. It's the new Russian Mullard. :icon_biggrin:
Staying with the budget mentality, I've installed a $30 Celestion. Still waiting on the grill cloth.
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> EL84 cathode pin 3 - 7.77V
> EL84 plate pin 7 - 243.9V
> Current draw - 51.8MA
> Dissipation - 12.231 watts
At least 10% of EL84 cathode current goes to screen, not plate. I get 11.1 watts of plate dissipation.
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Thanks PRR, you just made my amp "perfect."
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I'll get a faceplate made next. Maybe something like this.
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Late last night when it was really quiet, I noticed that if I let a B flat on the sixth fret of the low E string ring out it would start to oscillate as it faded out.
Well, I woke up bothered by this. So, I moved the ground wire from the EF86 pin 2 straight to the input jack ground (it was connected to the board at the negative end of the cathode cap, and shared a ground wire from there), and I replaced the 1Meg resistor coming off of B+ to pin 1 of the EF86 with the 5.6Meg resistor shown on the Vox schematic. I also rearranged the red wire from pin 6 so that it is tucked down and does not cross other wires.
This almost completely eliminated the wavering B flat, and the low level hum is reduced, too. Quiet enough that it I have to stop playing and listen for it. I might also add some adhesive foil under the top of the cabinet since the chassis is open on top.
Finally, I ordered some better tubes. I had grabbed the cheapest ones originally, but after some research I found that there are some better EF86's than the Electro Harmonix, and better EZ81's than the JJ.
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Grill cloth and custom faceplate installed. For a "first-time" amp build I'm happy with it.
I'd put my name on it. :icon_biggrin:
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Looks Nice :thumbsup:
Franco
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Very tidy
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Thanks Franco!
Thanks tubeswell!
I still need to add the piping around the grill to cover up the small gaps.
I made some minor changes and the amp is now as good as I think I can get it.
I switched back to the 1Meg screen resistor from the 5.6Meg; I probably should have just left that alone. But, I have to tinker with stuff.
I did some research on filter capacitance and rectifier cathode coatings and I'm confident that I'm safe in these changes that I made. I was still getting some hum, so I bumped the first filter cap up from 30uf to 47uf, the second is the same at 30uf, and I replaced the third 8uf with a 10uf of the same brand as the others (F&T). I changed that one because I think I might have damaged it during soldering.
Now the amp is very quiet.
I replaced the JJ EZ81 and the Electro-Harmonix EF86 with a Tesla EZ81 and a Tung-Sol EF806 SG.
The amp now sounds better than ever, and it's very quiet. I like it a lot; it pairs with my Alnico II Tele pickups really well.
So, a genuine thank you to everyone who showed an interest, shared their knowledge and guided me through this. And, I didn't get zapped. (I did notice that warm tubes seem to drain the caps when the power is switched off, but I still connected my discharge tool to develop the habit.)
Wow, I'm ready to start my next build...
Sluckey, I'm thinking your Trem-O-Nator would be good with your AB763 Deluxe Lite.
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Wow, I'm ready to start my next build...
call 1800gethelp if this becomes an addiction :icon_biggrin:
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Wow, I'm ready to start my next build...
call 1800gethelp if this becomes an addiction :icon_biggrin:
what's that again? guitar amps? <click> buzzzzzzzz (dial tone).
--pete
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Yeah, I might be nearing the slippery slope. I've actually been thinking about what I can sell off on eBay from my "real money pit" hobby, radio control airplanes, to fund another amp project. There's some money invested there, after 30yrs of collecting airplane stuff.